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Genome sequencing of the nine-spined stickleback (Pungitius pungitius) provides insights into chromosome evolution.

By Srinidhi Varadharajan, Pasi Rastas, Ari Löytynoja, Michael Matschiner, Federico C. F. Calboli, Baocheng Guo, Alexander J. Nederbragt, Kjetill S Jakobsen, Juha Merilä

Posted 21 Aug 2019
bioRxiv DOI: 10.1101/741751 (published DOI: 10.1093/gbe/evz240)

The Gasterostidae fish family hosts several species that are important models for eco-evolutionary, genetic and genomic research. In particular, a wealth of genetic and genomic data have been generated for the three-spined stickleback ( Gasterosteus aculeatus ), the 'ecology's supermodel', while the genomic resources for the nine-spined stickleback ( Pungitius pungitius ) have remained relatively scarce. Here, we report a high-quality chromosome-level genome assembly of P. pungitius consisting of 5,303 contigs (N50 = 1.2 Mbp) with a total size of 521 Mbp. These contigs were mapped to 21 linkage groups using a high-density linkage map, yielding a final assembly with 98.5% BUSCO completeness. A total of 25,062 protein-coding genes were annotated, and ca. 23% of the assembly was found to consist of repetitive elements. A comprehensive analysis of repetitive elements uncovered centromeric-specific tandem repeats and provided insights into the evolution of retrotransposons. A multigene phylogenetic analysis inferred a divergence time of about 26 million years (MYA) between nine- and three-spined sticklebacks, which is far older than the commonly assumed estimate of 13 MYA. Compared to the three-spined stickleback, we identified an additional duplication of several genes in the hemoglobin cluster. Sequencing data from populations adapted to different environments indicated potential copy number variations in hemoglobin genes. Furthermore, genome-wide synteny comparisons between three- and nine-spined sticklebacks identified chromosomal rearrangements underlying the karyotypic differences between the two species. The high-quality chromosome-scale assembly of the nine-spined stickleback genome obtained with long-read sequencing technology provides a crucial resource for comparative and population genomic investigations of stickleback fishes and teleosts.

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